The molecular mechanisms by which cells control cytoskeletal organization in response to extracellular cues, such as growth factors or other cells, are not understood. While the mammalian Rho-related GTP-binding proteins Rac and Cdc42 have been shown to function in the regulation of actin assembly and cell polarity, the downstream targets that mediate these effects on the cytoskeleton are not known. Several candidate molecules including PAK protein kinase have been proposed to link these GTPases to components which control cytoskeletal organization. PAK is a member of a growing family of kinases which includes two closely related homologs of Saccharomyces cerevisiae: STE2O, which functions in the yeast pheromone response pathway and cytoskeletal organization, and CLA4, which has functions in morphogenesis including budding and cytokinesis. The main objective of this proposal is to investigate the role of PAK in mammalian cells. By taking advantage of the highly conserved MAP kinase cascades of S. cerevisiae, we have identified constitutively active alleles of PAK. These activated alleles will be used to study the effects of PAK on the mammalian cytoskeleton and intracellular signaling pathways. In addition, we plan to identify the components which function downstream of PAK in cytoskeletal control. Understanding the relationship between RacliCdc42, PAK, and the cytoskeleton will contribute to our understanding of how signaling via Rho-like GTPases elicits control of the cytoskeleton to regulate aspects of mammalian cell growth and development.